Semiconductor device with field threshold MOSFET for high voltage termination

ABSTRACT

This invention discloses a semiconductor power device disposed in a semiconductor substrate comprising a lightly doped layer formed on a heavily doped layer and having an active cell area and an edge termination area. The edge termination area comprises a plurality P-channel MOSFETs. By connecting the gate to the drain electrode, the P-channel MOSFET transistors formed on the edge termination are sequentially turned on when the applied voltage is equal to or greater than the threshold voltage Vt of the P-channel MOSFET transistors, thereby optimizing the voltage blocked by each region.

This patent application is a Divisional application and claim thePriority Date of a co-pending application Ser. No. 13/135,985 filed bythe Applicants of this application on Jul. 19, 2011. The Disclosuresmade in application Ser. No. 13/135,985 are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the semiconductor power devices. Moreparticularly, this invention relates to configurations and methods formanufacturing of new and improved edge terminations for high voltage(HV) devices for improved reliability and to reduce the areas occupiedby the termination areas while maintaining high breakdown voltages.

2. Description of the Prior Art

Conventional floating guard rings in the termination area are notreliable especially products housed in plastic packages due to spacingof the Floating Guard Rings (FGR) and the charge from dielectric layerunder the metal layer, passivation film, and/or charge from the plasticmolding compounds. Other high voltage (higher than 400V) terminationstructures, such as Junction Termination Extension (JTE), Field GuardRing and Field Plate (FGR-FP), minimize sensitivity of FGR type highvoltage (HV) termination.

“Optimization and Surface Charge Sensitivity of High Voltage BlockingStructures with Shallow Junctions” by the inventor, Hamza Yilmaz,published in IEEE Transactions on Electron Devices, vol. 38, No. 3, July1991, pp. 1666-167, discloses a high voltage blocking and terminationstructure using offset (optimum) multiple field plate and field-limitingring (OFP-FLR) structure and optimized multiple zone JTE (MZ-JTE)structure to improve the breakdown voltage without additionalpassivation and process complexities. In the OFP-FLR structure, eachfield limiting ring has a field plate covering most of the surface spacebetween two adjacent field limiting rings located at the surface of thesilicon substrate with a gap between two field plates. A multiple zoneJTE structure includes multiple lightly p-doped zones located at thesurface of the silicon substrate and next to each other with the firstp-JTE is next to the p+ electrode.

“Junction termination extension (JTE), A New Technique for IncreasingAvalanche Breakdown Voltage and Controlling Surface Electric Fields inP-N Junctions” by Victor A. K. Temple, published in InternationalElectron Devices Meeting, 1977, PP. 423-426, discloses a method offorming a junction termination extension to improve the breakdownvoltage using implantation rather than by shaping or etching an alreadypresent semiconductor substrate. The advantages of this method areimmediately apparent-control over the actual dopant charge to onepercent accuracy and added flexibility in that the implant step can bedone almost anywhere during processing.

U.S. Pat. No. 6,011,298 discloses a high voltage termination structurewith buried field-shaping region for increasing a breakdown voltage. Thetermination structure includes a buried field-shaping region, such as aburied field-shaping ring, separated from and beneath the device regionwith a distance sufficient to permit a depletion region to form betweenthe buried field-shaping region and the device region when a firstvoltage is applied between the device region and the substrate and toproduce a larger radius of curvature of the depletion region formedabout the device region when a second voltage that is larger than thefirst voltage is applied between the device region and the substrate.

U.S. Pat. No. 4,158,206 discloses semiconductor device, which includes abody of semiconductor material having a PN junction terminating at amajor surface, with buried field limiting rings formed within the bodyand extending around a portion of the PN junction. Buried field limitingrings reduce the electric field intensity at the surface intercept ofthe reverse biased PN junction, thus increase the reverse bias voltagesustainable by the PN junction of interest and increase the breakdownvoltage of the semiconductor material.

However conventional FGR-FP does not completely shield surface of the HVtermination region from charges from wafer surface passivation filmsand/or assembly and package material (i.e., molding compound andassembly site).

Therefore a need still exists to provide a termination configurationthat completely seals the surface of the HV termination region with polySilicon or metal gate MOSFET structures.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide a new andimproved edge termination configuration to reduce the electrical fieldcrowding effects near the blocking junction at the device edge andprovide a compact termination with lower surface electric field that isless sensitive to surface charge. This is achieved with the formation ofa plurality of P-channel MOSFETs in the N region and between two P-typediffusion regions or between two floating guard rings (FGRs). In oneembodiment, a poly silicon or metal layer covering an oxide layer aredeposited at the region between the FGRs. This poly silicon or metallayer functions as a planar gate for the P-channel MOSFET transistors.In another embodiment, a gate material filled in a trench functions as atrench gate for the P-channel MOSFET transistor. By connecting the gateto the drain electrode, the P-channel MOSFET transistors act as a chainin the termination area, between the active area and the scribe region(die edge), in a cascade fashion. Threshold voltage of the P-channelMOSFET will determine the level of the potential of each floating guardring. This new HV termination structure can be applied to planar as wellas Trench based HV devices.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a layout of a high voltage (HV) device.

FIG. 2 is a cross-sectional view of the HV device described in FIG. 1along the line A-A according to a first embodiment of the presentinvention.

FIG. 3 is a cross-sectional view illustrating an alternativeconfiguration of the HV device described in FIG. 2.

FIG. 4 is a cross-sectional view of the HV device described in FIG. 1along the line A-A according to a second embodiment of the presentinvention.

FIG. 5 is a cross-sectional view of the HV device described in FIG. 1along the line A-A according to a third embodiment of the presentinvention.

FIG. 6 is a plot illustrating the electric potential distribution on thesurface of the silicon at the termination area of a HV device.

FIG. 7 is a plot showing a simulation of electric potential distributionon the surface of the silicon at the termination area of a HV device.

FIG. 8 is an image showing the potential lines in the silicon substrateat the termination area.

FIGS. 9A-9B are cross-section views showing the HV terminationstructures with field plates and a channel stop region for HV devices.

DETAILED DESCRIPTION OF THE METHOD

FIG. 1 is a top view of a layout of a HV device 100, such as a HV MOSFETor a HV IGBT, which includes an active area 102 and a termination area104. The active area 102 of high voltage (HV) device 100 includes aplurality of either planar gate HV MOSFETs/IGBTs or trench gate HVMOSFETs/IGBTs 106. The termination area 104 includes a plurality ofP-channel MOSFETs 108 connected in series, referred to as fieldthreshold MOSFETs. The field threshold MOSFETs are formed in terminationarea to sustain a high breakdown voltage.

FIG. 2 is a cross-sectional view of a planar gate HV device 200 of thetype depicted in FIG. 1 along line A-A according to a first embodimentof the present invention. The HV device 200 is formed on a semiconductorsubstrate including a lightly doped N-type layer 205 supported on top ofa heavily doped layer 105. The doping polarities of heavily doped layer105 and the lightly doped layer 205 are the same for a HV MOSFET deviceand are opposite for a HV IGBT device. For simplicity the description inthis application only illustrates a HV MOSFET device. The HV device 200includes an active area 201 and a termination area 203. The active area201 includes a plurality of planar gate N-channel vertical MOSFETs 202,each of which includes an n+ source 204, a drain formed in the n+substrate 105 and a planar gate 206. The termination area 203 includes aplurality of P-channel MOSFETs 210, each of which locates between twoP-type diffusion regions or two floating guard rings (FGRs) 212. TheP-type floating guard rings have a heavier doping concentration than thelightly doped N-type layer 205. The plurality of termination FGRs areformed to have a distance ranging from 1 to 10 microns between twoadjacent termination FGRs and a depth into the semiconductor substrateranging from 0.5 to 8 microns. Each P-channel MOSFET 210 contains aconductive layer such as doped polysilicon or metal layer 214 insulatedfrom the semiconductor substrate by an oxide layer 215, which isdeposited at the region between two FGRs 212 forming a planar gatelateral MOSFET with the two FGRs 212 on each side of the of thepolysilicon or metal layer 214 functioning as the source and drain ofthe lateral P-channel MOSFET. This poly silicon or metal layer 214functions as a planar gate for the P-channel MOSFET 210. Each FGR 212between two gate sections of polysilicon or metal layer 214 intermination area 203 functions as the drain of one P-channel MOSFET 210and the source for the other P-channel MOSFET. The drain and sourceregions disposed on two opposite sides of the gate section having adopant concentration ranging from 1e17 cm⁻³ to 1e20 cm⁻³ By connectingthe gate 214 to the drain electrode 212 of the P-Channel MOSFET (Drainof the P-Channel MOSFET is the p-region with lower Potential), theP-channel MOSFET transistors 210 act as a chain in the termination area203 in a cascade fashion. Threshold voltage of the P-channel MOSFET willdetermine the level of the potential of each floating guard ring (FGR)and can be adjusted by using a surface implant, for example n-typeimplant, to alter the doping of the top region 208. The FGR 212 may beformed at the same time the P body region of the active transistor isformed with the same dopant density and the gate 214 of lateral MOSFETin termination area may be formed in the same process of forming activetransistor planar gate. A high dope contact implant may be formed in theFGR 212 in the same way as the contact implant in active area forimproving the electrical contact to the FGR 212.

FIG. 3 is a cross-sectional view illustrating an alternativeconfiguration of the HV device 200 described in FIG. 2. The device 300of FIG. 3 is similar to the device 200 except that the P-channel MOSFETs211 in the termination area 303 of the device 300 also include n+ dummysource regions 204 that are not blocked in the termination area 303 inthe process of source implant in active area. The P-channel MOSFETs 211are functioning as the same way as in P-channel MOSFET 210 shown in FIG.2. Furthermore, the n+ dummy source regions 204 also configure theplanar gate MOSFETs as in termination area as N-channel vertical MOSFETswith the gates shorted to the source/body regions therefore never turnon.

The new HV termination structure of the present invention using thefield threshold MOSFETs as shown in FIGS. 2-3 also can be applied toTrench gate HV device as shown in FIGS. 4-5.

FIG. 4 is a cross-sectional view illustrating a trench gate high voltage(HV) device 400 according to a second embodiment of the presentinvention. The active area 401 of the device 400 includes a plurality oftrench gate vertical N-channel MOSFETs 402, each of which includes an n+source 404, a trench gate 406 and a drain formed in the n+ substrate105. Similar to device 200 of FIG. 2, the termination area 403 of thedevice 400 includes a plurality of P-channel planar gate lateral MOSFETs210, each of which locates between two P-type diffusion regions orbetween two FGRs 212 with the gate 214 connecting to the drain electrode212. The P-channel MOSFET transistors 210 act as a chain between theactive area and the scribe region (die edge) in a cascade fashion asdescribed above. The FGR 212 may be formed at the same time the P bodyregion of the active transistor is formed with the same dopant density.Alternatively, the termination area 403 can includes a plurality ofP-channel MOSFETs 211 as shown in FIG. 3.

FIG. 5 is a cross-section view illustrating another trench gate HVdevice 500 according to a third embodiment of the present invention. Inthis embodiment, the active area 501 includes a plurality of trench gatevertical N-channel MOSFETs 402 of the type depicted in FIG. 4. Thetermination area 503 includes a plurality of trench gate P-channellateral MOSFETs 504, each of which has the same structure as the trenchgate MOSFET 402 and locates between two P-type diffusion regions or twoFGRs 512. The plurality of termination trenches are formed to have adistance ranging from 0.5 to 5 microns between two adjacent terminationtrenches and a depth into the semiconductor substrate ranging from 0.5to 8 microns. The gate material 506 of each P-channel MOSFET 504functions as a trench gate and is connected to a drain electrode 512(Drain of the Trench MOSFET is the p-region with more negativePotential). Each FGR 512 between two trench gate 506 in termination area503 functions as the drain of one P-channel MOSFET 504 and the sourcefor the other P-channel MOSFET. By connecting the gate to the drainelectrode, the P-channel MOSFET transistors 504 act as a chain in thetermination area in a cascade fashion as described above. Thresholdvoltage of the trench MOSFET will determine the level of the potentialof each floating guard ring (FGR) and can be adjusted by using implant,for example n-type implant, or changing the doping concentration duringthe epitaxial grown of the region 508. The embodiment shown in FIG. 5shows the threshold adjustment layer 508 extending into the active area501. In another embodiment (not shown) the threshold adjustment layer508 may be formed through implant in the termination area only withoutextending into the active area 501. The threshold adjustment layer 508has a dopant concentration higher than the epitaxial layer 205. In oneembodiment the threshold adjustment layer 508 has a dopant concentrationhigher than the P body region of active area. The FGR 512 may be formedat the same time the P body region of the active transistor is formedwith the same dopant density. A high dope contact implant may be formedin the FGR 512 in the same way as the contact implant in active area forimproving the electrical contact to the FGR 512.

In order to implement each of these P-channel MOSFETs as a fieldthreshold MOSFET, instead of leaving the gates to have a floatingvoltage or shorting the gates to the source, in which the P-channelMOSFET is never turned on; each gate is connected to its correspondingdrain electrode thus the gate electrode and the drain electrode are samepotential. When a threshold voltage Vt is applied to the device, i.e.,Vds=Vgs=Vt, where Vds is the drain to source voltage, Vgs is the gate tosource voltage, each of these P-channel MOSFET is turned on. As thevoltage applied to the power device increases, these plurality ofP-channel MOSFETs are sequentially turned on to sustain graduallyincreasing voltage applied to the device. The potential climbs upuniformly at a field threshold voltage, for example 50V, per P-channelMOSFET or per trench. Therefore the number of P-channel MOSFETs neededdepends on the design breakdown voltage of the HV device. Typically,there are about 1 to 25 P-channel MOSFETs formed in an edge terminationarea of a width ranging from 5 microns to 250 microns. The fieldthreshold based termination with these multiple P-channel MOSFETs cantherefore sustain high breakdown voltage. FIG. 6 is a plot showingpotential distribution on the surface of the silicon substrate with anumber of P-channel MOSFETs formed at the termination area. FIG. 7 is aplot showing a simulation of the electric potential distribution at thesurface of the silicon substrate at the termination area. FIG. 8 is animage showing potential lines flattering in the silicon substrate thusreducing the electric field crowing and significantly increases thebreakdown voltage without requiring large termination area.

Absolute value of the threshold voltage of P-Channel MOSFET can beoptimized to achieve high breakdown voltage with optimum HV terminationsize. Low Vt requires more P-channel MOSFETs and thus the terminationarea becomes larger. In contrast, high Vt will not yield desiredbreakdown voltage at the termination area and breakdown voltage of theDevice will be lower than a target. Vt of the P-channel MOSFETs can beadjusted with the required high breakdown voltage specification byadjusting oxide thickness, or localized surface concentration adjustmenteither increasing N-type doping concentration to increase Vt, ordecrease the Vt by counter doping the N-type region's concentration. Thethreshold voltage Vt is ranging from 0.5 to 80 volts.

FIGS. 9A-9B are cross section views illustrating the end portions of theHV termination structures of the type depicted in FIGS. 2 to 5 combiningwith a first field plate formed adjacent to the last P-type diffusionregion or last floating guard rings and a field plate formed at thechannel stop region to further expand high voltage blocking capabilityof the edge termination.

As shown in FIGS. 9A-9B, a first field plate 904 is formed adjacent tothe last P-type diffusion region or last floating guard ring 212 asshown in FIGS. 3 and 5. The first field plate 904 extends from the lastfloating guard ring 212 towards the scribe line 920. The first fieldplate 904 is electrically connected to the last floating guard ring 212through a metal 910. A heavily doped channel stop region 902 of the sameconductivity type of the lightly doped layer of the semiconductorsubstrate, e.g., N+ doped region 902, is formed at the surface of thesemiconductor substrate near the edge of the termination area forstopping the electric field at the surface of the semiconductorsubstrate. A second field plate 905 is also formed adjacent to thechannel stop region 902 and extends from the channel stop region 902towards the active area. The second field plate 905 and the channel stopregion 902 are electrically connected to each other by metal 912. Thefield plates 904 and 905 are electrically isolated from the siliconsubstrate by a field oxide layer 906 and are isolated from each other bya Borophosphosilicate glass (BPSG) layer 908. The field plates 904 and905 spread the electric field at the edge of the termination area, thusincreasing the breakdown voltage.

The HV device termination structure of the present invention can also beapplied to many types of high voltage devices, including MOSFET, IGBT,JFET/SIT N-drift Diode type structures. The embodiments only illustrateN Channel devices. P Channel devices may be implanted by switching thedoping polarity types.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

We claim:
 1. A semiconductor power device disposed in a semiconductorsubstrate comprising a lightly doped layer formed on top of a heavilydoped layer and having an active cell area and an edge termination areawherein: said edge termination area comprises a plurality of terminationtrenches formed in said lightly doped layer and lined with a dielectriclayer and filled with a conductive material therein; and a plurality ofseries connected MOSFET transistors, each of which comprises a trenchgate region, a drain and source regions disposed on two opposite sidesof each of said termination trenches, with said conductive material ineach of said termination trenches functions as a trench gate for each ofsaid MOSFET transistors, wherein each trench gate is electricallyconnected to said drain region.
 2. The semiconductor power device ofclaim 1 wherein: said plurality of MOSFET transistors comprising aplurality of P-channel MOSFET transistors.
 3. The semiconductor powerdevice of claim 1 wherein: one of the plurality of MOSFET transistors isturned on when the applied voltage is greater than or equal to athreshold voltage of said MOSFET transistor, wherein said thresholdvoltage ranging from 0.5 to 80 volts.
 4. The semiconductor power deviceof claim 1 wherein: said edge termination has a width ranging from 5microns to 250 microns to form between 1 to 25 termination trenches insaid edge termination.
 5. The semiconductor power device of claim 1wherein: said plurality of termination trenches are formed to have adistance ranging from 0.5 to 5 microns between two adjacent terminationtrenches.
 6. The semiconductor power device of claim 1 wherein: saidplurality of termination trenches are formed to have a depth opened intosaid semiconductor substrate ranging from 0.5 to 8 microns.
 7. Thesemiconductor power device of claim 1 wherein: said drain and sourceregions disposed on two opposite sides of each of said terminationtrenches having a dopant concentration ranging from 1e17 cm⁻³ to 1e20cm⁻³.
 8. The semiconductor power device of claim 1 wherein: saidthreshold voltage of each MOSFET transistor is adjusted by localizedvariation of trench dielectric thickness and/or localized variation ofdoping concentration of said lightly doped layer of semiconductorsubstrate.
 9. The semiconductor power device of claim 1 furthercomprising: a first field plate starting from a vicinity of a lastfloating guard ring and expanding towards scribe lines: a heavily dopedchannel stop region; and a second field plate formed adjacent to saidchannel stop region and expanding towards said active cell area.
 10. Thesemiconductor power device of claim 1 further comprising: a first fieldplate starting from a vicinity of a last floating guard ring expandingtowards scribe lines; a heavily doped channel stop region with the sameconductivity type as said lightly doped layer.
 11. The semiconductorpower device of claim 10 further comprising: a second field plate formedadjacent to said channel stop region and expanding toward said activecell area.
 12. The semiconductor power device of claim 1 wherein: saidheavily doped layer has opposite polarity compare to lightly doped layerto form an IGBTs.
 13. The semiconductor power device of claim 1 wherein:said heavily doped layer has the same polarity compare to lightly dopedlayer to form MOSFETs.